1. Field of the Invention
This invention relates to semiconductor manufacturing, and more particularly to measuring surface curvature of semiconductor wafers.
2. Background of the Related Art
Integrated circuits are formed on semiconductor wafer substrates by a number of processing steps. These steps include deposition, etching;, implantation, doping, and other semiconductor processing steps well known to those skilled in the art.
Thin films are typically formed on wafer surfaces by a deposition process. The thickness of such films usually ranges from about a few hundred angstroms to several micrometers. Often, three or more film layers are formed on the surface of a single semiconductor wafer.
In the art of fabricating semiconductor wafers, it is of known importance to minimize or control stresses in surface films. High surface stresses can cause, for example, silicide lifting, the formation of voids or crack and other conditions that adversely affect semiconductor devices (i.e. chips) which are fabricated on the wafers. In practice, surface stresses become more problematical as the level of circuit integration increases, and are especially troublesome when fabricating large scale integration (LSI), very large scale integration (VLSI), and ultra large scale integration (ULSI) semiconductor devices.
The stress in the surface film of a semiconductor wafer can be either compressive or tensile. Assuming the film is on top of the wafer, a compressive stress in a surface film will cause a wafer to slightly bow in a concave direction, while a tensile stress in a surface film will cause a wafer to slightly bow in a concave direction. Therefore, both compressive and tensile stresses cause the surface of the semiconductor wafer to deviate from exact planarity. The extent of the deviation from planarity can be expressed in terms of the radius of curvature of a wafer surface. In general, the greater the magnitude of surface stress, the smaller the radius of curvature.
Because of the problems that can be caused by stresses in surface films on semiconductor wafers, it is highly desirable to be able to measure such stresses. The measurements can be used, for example, to identify wafers that are likely to provide low yields of semiconductor devices or which might produce devices prone to early failure. In normal practice, stresses in surface films are not measured directly but, instead, are inferred from measurements of the radius of curvature of the surface of interest.
A system for measuring the curvature of a workpiece, such as a wafer, is described in copending patent application Ser. No. 07/876,576, filed on behalf of David Cheng, entitled "Method and Apparatus for Measuring Surface Topography", now U.S. Pat. No. 5,270,550, which was incorporated herein by reference. This system reflects a guided beam of radiant energy, such as one generated by a laser, from a surface film of a workpiece. A detector detects a portion of the reflected beam; as the wafer or laser is moved, the deviation of the beam from a point at the detector is recorded and analyzed to detect curvature of the surface.
A problem encountered with this system is that the amplitude of the beam of energy reflected from the surface film of the workpiece can be reduced due to destructive interference. The interference is caused by reflection from the surface film on the workpiece, which includes an upper surface boundary and a lower surface boundary. A beam of light is partially reflected and partially transmitted through the upper film boundary. The transmitted portion of the beam is reflected by the lower film boundary and interferes with the first reflected portion of the beam due to well-known optical interference principles. The thickness of the surface film can cause the second reflected portion of the beam to be out-of-phase with the first reflected portion, and destructive interference in the entire beam can result. Destructive interference may weaken or almost completely cancel the amplitude of the reflected beam of energy, causing difficulties in detecting the reflected beam and resulting in errors in the curvature testing process.
In U.S. Pat. No. 5,134,303, by Blech et al., a dual frequency laser apparatus for measuring stress in a thin film is disclosed. A laser beam composed of two different wavelengths is directed onto a surface with a thin film and reflected to a detector. If one of the wavelengths permits destructive interference to occur in the reflected beam, the other transmitted wavelength may not, and the reflected beam can be detected. Two separate laser beams, each of a distinct wavelength, are combined into the dual-wavelength beam by a beamsplitter to accomplish this goal.
A problem with the prior art dual frequency laser apparatus is that a beam of multiple wavelengths is directed at a thin film surface. Both laser sources are required to concurrently project energy in a beam of multiple wavelengths, requiring both lasers to be concurrently powered and maintained. A waste of energy is thus evident.
What is needed is an apparatus and method that will reduce the problem of destructive interference that occur in surface film curvature measurement and also reduce the inefficiency of using combined, multiple-frequency energy sources to measure thin films.